System and method for programmable polarization-independent phase compensation of optical signals

ABSTRACT

A system and method for programmable phase compensation of optical signals is disclosed. The systems and methods include the use of a polarization-independent spatial light modulator (PI-SLM) so that the state of polarization (SOP) of the incoming optical signal need not be known. The system includes a first dispersive module that spatially separates the optical signal into its frequency components. The frequency components are spread over the active area of the PI-SLM. The active area of the PI-SLM includes an array of independently programmable addressable regions capable of altering the phase of the light incident thereon. An exemplary application of the invention is chromatic dispersion compensation. By knowing the amount of chromatic dispersion in the optical signal, or alternatively, by knowing the amount of chromatic dispersion to be introduced into the optical signal downstream, the appropriate phase adjustments can be made to each frequency component of the signal. The phase-adjusted frequency components are then recombined via a second dispersive module to form a compensated optical signal.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional under 37 CFR 1.53(b) of U.S.application Ser. No. 10/178,949 filed Jun. 24, 2002, which claims thebenefit under 35 U.S.C. 119(e) of U.S. Provisional Application Ser. No.60/303,763 filed Jul. 6, 2001, which applications are incorporatedherein by reference.

FIELD OF THE INVENTION

The present invention relates to optical communications and theprocessing of optical signals, and in particular relates to systems andmethods for adjusting the phase of optical signals having an arbitrarypolarization.

BACKGROUND OF THE INVENTION

The transmission of information over optical fibers is becomingpervasive. This is motivated, at least in part, because optical fiberoffers much larger bandwidths than electrical cable. Moreover, opticalfiber can connect nodes over large distances and transmit opticalinformation between such nodes at the speed of light.

There are, however, a number of physical effects that limit the abilityto transmit large amounts of information over an optical fiber. One sucheffect is called “chromatic dispersion,” which refers to the spreadingof a pulse of light (i.e., an “optical signal” or “lightwave signal”)due to the variation in the propagation velocity of the differentoptical frequencies (or equivalently, wavelengths) making up the pulse.

Chromatic dispersion has two root causes. The first is due to the factthat silica of the optical fiber, like any optical material, has anindex of refraction that is frequency-dependent. This is referred to as“material dispersion.” The second cause is due to the nature of thepropagation of light down the fiber and is referred to as “waveguidedispersion.” The power distribution of the light between the core andthe cladding of the fiber is a function of frequency. This means the“effective index” or “propagation constant” of the waveguide mode is afunction of frequency as well, which causes the optical signal todisperse as it travels down the fiber.

In optical fiber communication systems, chromatic dispersion causesindividual bits to broaden, since each bit is composed of a range ofoptical frequencies that separate due to their different propagationvelocities. Such broadening eventually leads to intersymbol interferencedue to overlap of adjacent bits, which results in unacceptable datatransmission errors. Chromatic dispersion compensation is usually neededto obtain the required performance in lightwave transmission systemsoperating at per channel data rates of 10 Gb/s or above. For example,the dispersion of a standard single mode fiber (SMF) at the keylightwave communications wavelength of 1550 nm is roughly 17 ps/nm-km.For a 10 Gb/s transmission system, the optical bandwidth per channel istypically a minimum of 0.1 nm, and is often greater. Transmissionthrough a 30 km span of SMF would lead to a chromatic dispersivebroadening of the signal of 51 ps, which is 50% of the bit period (100ps).

Such a broadening is unacceptably large and would lead to a large errorrate. The problem becomes much more acute with higher data rates, suchas 40 Gb/s per channel systems currently under development. The problemwill even become more acute for the anticipated higher data rate systemspresently being contemplated. Further details about the nature ofchromatic dispersion in optical fibers and the consequences for opticalnetworks can be found in the book by Ramaswami and Sivarajan, entitledOptical Networks, a Practical Perspective, Morgan Kaufmann Publishers,in chapter 2.3.

Efforts have been made in the past to develop systems and methods forcompensating for the effects of chromatic dispersion. For example,dispersion-compensating fibers (DCF) have been developed that have theopposite sign of dispersion compared to conventional single mode fibershave been developed and are widely deployed as compensators. However,the DCF technique lacks the ability to easily fine tune the spectralvariation of the dispersion and involves a relatively large insertionloss for long fiber links. Chirped fiber Bragg gratings can alsocompensate fixed amounts of dispersion, but only for one WDM channel ata time. Both techniques lack the ability to reprogram or programmablyfine tune the amount of dispersion and its spectral profile, which islikely to be needed to develop higher rate lightwave communicationsystems.

A number of workers have used programmable pulse shapers to programmablycompensate chromatic dispersion in high-power femtosecond pulseamplifiers and in nonlinear optical pulse compression systems. A varietyof spatial light modulator (SLM) types have been used, including liquidcrystals, acousto-optic modulators, and deformable mirrors.

By way of examples, the use of a deformable-mirror SLM to correctchromatic dispersion is described in the paper by E. Zeek et al., Pulsecompression by use of deformable mirrors, Opt. Lett, 24, 493-495 (1999).The use of an arrayed waveguide grating (AWG) rather than a bulkdiffraction grating as the spectral disperser is described in the paperby H. Takenouchi et al., entitled 2×40-channel dispersion-slopecompensator for 40-Gbit/s WDM transmission systems covering entire C-and L-bands, presented at the Optical Fiber Communications Conference(OFC), sponsored by the Optical Society of America, Anaheim, Calif.,March 2001; however, in this paper a fixed phase mask is used in placeof an SLM, with the result that the dispersion is not programmable.Further, the article by C. Chang et al. entitled Dispersion-free fibertransmission for femtosecond pulses by use of a dispersion-compensatingfiber and a programmable pulse shaper, Opt. Lett. 23, 283-285 (1998)describes chromatic dispersion compensation using a liquid crystal SLM.

These and the other efforts described in the cited references all havethe shortcoming that the operation of the dispersion compensation systemdepends on the SOP and/or that the system is not sufficientlyprogrammable to handle the dispersion slope and higher-order dispersionterms or to reprogram the dispersion profile to accommodate changes inthe length of optical fiber links in a switched optical networkingenvironment. The dependence of a chromatic dispersion compensationsystem on the SOP of the input lightwave is major shortcoming becausethe SOP of light having traveled through an optical fiber system isscrambled and can vary with time, resulting in polarization-dependentloss (PDL). Further, the inability to robustly perform phase encoding ofthe signal reduces the ability to accurately compensate for thechromatic dispersion characteristics of a given optical fiber system.

Accordingly, what is needed is a system and method that can programmablycompensate, with a high degree of accuracy, an optical signal forchromatic dispersion effects of an optical fiber, while also beinginsensitive to the SOP of the light signal being processed.

SUMMARY OF THE INVENTION

The present invention relates to optical communications and theprocessing of optical signals, and in particular relates to systems andmethods for adjusting the phase of optical signals having an arbitrarypolarization. The present invention finds particularly utility incorrecting, reducing or otherwise adjusting chromatic dispersion inoptical signals.

The present invention provides the capability to programmably controlpulse broadening due to chromatic dispersion in chromatically dispersivemedia, and in particular in optical fiber communications systems andnetworks. This capability allows optical fiber lightwave communicationsystems to run at higher speeds over longer distances by compensatingchromatic dispersion, which is regarded as a key impairment forhigh-performance lightwave communication systems. The present inventioncan be applied both to very high-speed time-division multiplexed (TDM)and to wavelength division multiplexed (WDM) optical communications. Inthe case of WDM systems, several WDM channels can be independentlycompensated and can be programmed to achieve nearly arbitrary dispersionprofiles in order to match the system requirements. The chromaticdispersion compensator can handle input optical signals with arbitraryand unspecified state of polarization, and may be configured to providesubstantially zero PDL.

Accordingly, a first aspect of the invention is a system forprogrammably adjusting the phase of the frequency components of anoptical signal of arbitrary polarization. The system includes a firstdispersive module arranged to receive and disperse the optical signalinto its frequency components. A polarization-independent spatial lightmodulator (PI-SLM) having an active area comprising a plurality ofindependently programmable addressable regions is arranged to receivethe frequency components on the active area. The PI-SLM may be, forexample, a liquid-crystal SLM adapted for polarization-independentoperation, or a programmably deformable mirror. A controller is coupledto the PI-SLM. During operation of the system, the controller causes thePI-SLM to independently adjust the phase of one or more of the frequencycomponents.

In an example embodiment of the invention, the phase-adjustment isperformed to alter chromatic dispersion in the optical signal.

A second aspect of the invention is a method of programmably adjustingthe phase of the frequency components of an optical signal of arbitrarypolarization to adjust the amount of chromatic dispersion in the signal.The method includes spatially dispersing frequency components of theoptical signal onto a polarization-independent spatial light modulator(PI-SLM) over an active area having a plurality of independentlyprogrammable addressable regions. The method further includesindependently adjusting one or more of the addressable regions to alterthe phase of the corresponding frequency components incident thereon.The phase-altered signals are then recombined to produce a compensatedoptical signal.

A third aspect of the invention includes the above described method, andinvolves adjusting the polarization of the optical signal frequencycomponents so as to reduce any polarization-dependent loss (PDL) due todispersing the optical signal into its frequency components and/orrecombining the phase-altered frequency components to form thecompensated optical signal.

A fourth aspect of the invention involves using the chromatic dispersioncompensation system of the present invention to compensate for chromaticdispersion in channel optical signals in a wavelength-divisionmultiplexed (WDM) signal. This is accomplished by dividing up the activearea of the PI-SLM into sets of addressable regions corresponding to thefrequency components of the different channel optical signals, and thencompensating the frequency components of each channel signal. Thecompensated channel signals can then be detected, transferred to anotheroptical system, or recombined with a multiplexer to form a compensatedWDM signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the chromatic dispersion compensationsystem of the present invention as part of a larger lightwave opticalsystem for reducing chromatic dispersion in an optical signal caused,for example, by transmission through an optical system with chromaticdispersion;

FIG. 2A is a close-up perspective view of a liquid-crystal-based SLMthat is adaptable as one type of polarization-insensitive SLM (PI-SLM)suitable for use in the chromatic dispersion compensation system of FIG.1;

FIG. 2B is an exploded perspective view of a reflective single liquidcrystal layer PI-SLM that includes a polarization-adjusting elementbetween the liquid crystal array of addressable regions and thereflecting member;

FIG. 3 is a schematic diagram of a transmission-mode embodiment of thechromatic dispersion compensation system of FIG. 1 that employs thetwo-layer liquid-crystal SLM of FIG. 2A, which is adapted to bepolarization-independent, the system further including diffractiongratings in the dispersive modules;

FIG. 4 is a schematic diagram of an on-axis reflection-mode embodimentof the chromatic dispersion compensation system of FIG. 1 that employs asingle optical fiber as the input and output optical fiber, and acirculator connected to first and second optical fibers and the singleoptical fiber;

FIG. 5A is a schematic diagram of an on-axis reflection-mode embodimentof a chromatic dispersion compensation system similar to that of FIG. 4,but that includes an optical system having magnification to reduce theoverall size of the system;

FIG. 5B is a schematic diagram of an example telescope embodiment of themagnification optical system of the chromatic dispersion compensationsystem of FIG. 5A

FIG. 6 is a schematic diagram of an off-axis reflection-mode embodimentof the chromatic dispersion compensation system of FIG. 1 similar tothat of FIG. 4 and that employs input and output optical fibers;

FIG. 7A is a schematic diagram of an optical processing system thatincludes the chromatic dispersion compensation system of the presentinvention, wherein the latter is used to perform post-compensation of anoptical signal having passed through an optical system with chromaticdispersion;

FIG. 7B is a schematic diagram of an optical processing system thatincludes the chromatic dispersion compensation system of the presentinvention, wherein the latter is used to perform pre-compensation of anoptical signal to be passed through an optical system with chromaticdispersion;

FIGS. 7C and 7D are schematic diagrams of embodiments of opticalprocessing systems that includes the chromatic dispersion compensationsystem of the present invention, wherein a detection system is used tointerrogate the optical system to measure its chromatic dispersion andto provide information for the chromatic dispersion compensation systemto perform post-compensation (FIG. 7C) or pre-compensation (FIG. 7D) ofan optical signal;

FIG. 8A is an embodiment of an optical processing system that includesthe chromatic compensation system of the present invention, wherein thesignals associated with different WDM channels are individuallycompensated for chromatic dispersion and then received by respectiveoptical systems;

FIG. 8B is an embodiment of an optical processing system similar to thatof FIG. 8A, but further including a multiplexer connected to each of theoptical fibers that are connected to the respective chromatic dispersioncompensation systems, for multiplexing the compensated signals;

FIG. 9 is a close-up view of the first dispersive module and PI-SLM ofsystem of FIG. 1 as used to perform chromatic dispersion compensationfor a WDM signal having different channel optical signals, wherein thePI-SLM includes sets of addressable regions corresponding to thefrequency components associated with each channel optical signal.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to optical communications and theprocessing of optical signals, and in particular relates to systems andmethods for adjusting the phase of optical signals having an arbitrarypolarization. In the following detailed description of the embodimentsof the invention, reference is made to the accompanying drawings thatform a part hereof, and in which is shown by way of illustrationspecific embodiments in which the invention may be practiced. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the invention, and it is to be understood thatother embodiments may be utilized and that changes may be made withoutdeparting from the scope of the present invention. The followingdetailed description is, therefore, not to be taken in a limiting sense,and the scope of the present invention is defined only by the appendedclaims.

With reference to FIG. 1, there is shown a phase vs. frequencycompensation system 100 according to the present invention as part of anoptical processing system 106. System 100 is illustrated in transmissionmode for the sake of convenience and illustration, and one skilled inthe art will appreciate that there are associated folded reflection-modesystems that have the identical or analogous properties as representedin the transmission mode schematic diagram of FIG. 1. Such systems arediscussed below and shown in FIGS. 3-6.

System 100 can be used, for example, to compensate, reduce or otherwisealter the chromatic dispersion in an optical signal 110. As chromaticdispersion is a variation in the propagation velocity of the differentfrequency (or, equivalently, wavelength) components making up theoptical signal, chromatic dispersion can be adjusted by imparting anappropriate phase to one or more of the frequency components based on adesired phase vs. frequency relationship. The discussion of system 100and the various implementations of system 100 emphasizespolarization-independent chromatic dispersion compensation because thepresent invention is eminently suited to such a function. However, itwill be apparent to one skilled in the art that system 100 can performother polarization-independent phase vs. frequency adjustment functions,such as for example wavefront reconstruction, wavefront alteration, andpulse shaping.

Chromatic dispersion may be present in optical signal 110 and caused,for example, by the signal having passed through a first optical system120 having chromatic dispersion. Optical system 120 may include, forexample, a distance of optical fiber 122 having chromatic dispersion.Optical system 120 may also include other optical components, e.g.,laser sources, amplifiers, switches, gratings, routers, lenses, couplersetc., collectively shown as an element 124, that are capable ofintroducing additional amounts of chromatic dispersion. Optical system120 thus produces chromatic dispersion in optical signal 110 from one ormore sources that, absent compensation, limits the bandwidth and/orfidelity of optical processing system 106 as a whole. In particular,chromatic dispersion causes pulse-broadening that, absent compensation,sets an upper limit for the bit rate period because of intersymbolinterference.

A preferred consequence of compensating chromatic dispersion in opticalprocessing system 106 is that it can optimize the usable bandwidth ofoptical signal 110. For example, performing chromatic dispersioncompensation of optical signal 110 to form a compensated (i.e.,phase-adjusted) signal 126 may be necessary to successfully transmitinformation through a second optical system 130, which may itselfinclude sources of chromatic dispersion, such as an optical fiber 132 aswell as other sources 124 of chromatic dispersion.

With continuing reference to FIG. 1, system 100 includes in order alongan optical axis A1, a first dispersive module 136, and apolarization-independent spatial light modulator (PI-SLM) 140 having anactive region 144 comprising an array 145 of independently programmableaddressable regions 146. Also included downstream from PI-SLM 140 is asecond dispersive module 148. Dispersive module 136 serves to spatiallyseparate optical signal 110 into its frequency components 200 and directthese components onto active area 144 of PI-SLM 140. PI-SLM 140 iselectronically connected to a controller 150 that controls the operationof the PI-SLM, as described below.

PI-SLM 140 may be one of a number of spatial light modulators that donot depend on the polarization of the input light signal, and that donot impart a polarization to a light signal. More generally, as usedherein, a PI-SLM is any component or aggregation of components thatdefines an active area 144 having multiple, addressable regions 146 foradjusting the phase, and/or amplitude of light wavefronts incident onthe regions. For example, the PI-SLM can have multiple, independentlyaddressable regions such as a discrete array of independentlyaddressable addressable regions. Alternatively, the PI-SLM can havemultiple, addressable regions that partially overlap. For example, thePI-SLM can be a deformable mirror having multiple, addressable actuatorsthat deform overlapping regions of the active area. Furthermore, otherPI-SLMs can vary the phase, for example, by varying the refractive indexof the addressable regions. In the example embodiment shown in FIG. 1,SLM 140 is electronically addressable through its connection withcontroller 150. In other embodiments, however, the SLM may be opticallyaddressable. Dispersive module 136 directs frequency components 200 ontothe multiple regions of SLM 140 so that there is a known relationshipbetween each addressable region 146 and the particular frequencycomponent or frequency components 200 incident on that region.

Thus, the PI-SLM can adjust the phase, and/or amplitude of the incidentfrequency components by, e.g., reflection, transmission, diffraction, orsome combination thereof. As described further below, in manyembodiments, the PI-SLM involves one or more liquid crystal layers,whose birefringence and/or orientation are controlled to provide adesired series of adjustments for each SLM addressable region. Forexample, the liquid-crystal PI-SLM may include twisted nematic liquidcrystals, non-twisted nematic liquid crystals, and/or ferroelectricliquid crystals. In further embodiments, the PI-SLM can include aninorganic electro-optic modulator, e.g., a lithium niobate crystalcoupled to a generator providing a spatially addressable E-field, or anacousto-optic modulator coupled to a transducer providing a spatiallyaddressable acoustic wave.

In one example embodiment, PI-SLM 140 is a multi-layer liquid-crystalmodulator, such as described in U.S. Pat. No. 5,719,650 (the '650Patent), which patent is incorporated herein by reference. As describedin the '650 Patent and as illustrated in FIG. 2A, liquid-crystal PI-SLM140 includes in active region 144 first and second arrays 160 and 162 ofadjacent polarization rotating or adjustable birefringent elements(addressable regions) 166. Elements 166 are aligned along a first axisin array 160 and a along a second axis (preferably, 90-degrees withrespect to the first axis) in array 162.

In the present invention, liquid-crystal-based PI-SLM 140 of FIG. 2Aneeds to be adapted for use in system 100 so that it can be operatedwithout concern for polarization effects, and in particular, withoutregard to the polarization of optical signal 110.

Specifically, if the liquid crystal alignment directions (axes) ofarrays 160 and 162 are described as x and y, then the polarizationtransfer matrix M(ω) for the two-layered liquid crystal PI-SLM 140 isgiven by: $\begin{matrix}{{M(\omega)} = \begin{pmatrix}{\mathbb{e}}^{{\mathbb{i}}\quad{\phi_{x}{(\omega)}}} & 0 \\0 & {\mathbb{e}}^{{\mathbb{i}}\quad{\phi_{y}{(\omega)}}}\end{pmatrix}} & \left( {{EQ}.\quad 1} \right)\end{matrix}$wherein exp{iφ_(x)(107 )} and exp{iφ_(y)(ω)} are the phases shiftsimparted by the SLM for array 160 and 162, respectively.

Thus, to obtain the output electric field vector E_(OUT)(ω) from theinput electric field vector E_(IN)(ω) the following operation isperformed:E _(OUT)(ω)=M(ω)E _(IN)(ω)   (EQ. 2)

By setting φ_(x)(ω)=φ_(y)(ω), M(ω) becomes: $\begin{matrix}{{M(\omega)} = {\exp\left\{ {{\mathbb{i}}\quad{\phi_{x}(\omega)}} \right\}\quad\begin{pmatrix}1 & 0 \\0 & 1\end{pmatrix}}} & \left( {{EQ}.\quad 3} \right)\end{matrix}$

Thus, in the case of the liquid-crystal-based PI-SLM 140 of FIG. 2A,Equation 3 reveals that PI-SLM 140 can be arranged so that it does notchange the SOP of light passing therethrough. Thus, the phase shiftimparted by liquid-crystal PI-SLM 140 of FIG. 2A can be made independentof the polarization of the input electric field vector by aligning therespective liquid crystal axes of element 166 in array 160 and 162 at90-degrees with respect to one another.

With reference now to FIG. 2B, this effect can be achieved by areflective SLM having a single array of liquid crystal addressableregions, wherein light passes twice through elements 166 in single array160, and wherein a 90-degree polarization change is imparted to thelight prior to it passing back through the array. This can be achieved,for example, by providing a polarization-adjusting element 168 (e.g., awave-plate) between array 160 and reflective member 170 (e.g., amirror), wherein the polarization-adjusting element is designed toimpart a total of 90-degrees of polarization rotation upon the lightpassing twice through the element. As one example, this can be achievedby passing twice through a properly oriented quarter wave plate. As asecond example, this can be achieved by the use of a Faraday mirrorproviding a total of 90-degrees of polarization rotation. In the lattercase, polarization-adjustment element 168 and reflecting member 170 arecombined.

For a given SLM element 166, both φ_(x)(ω) and φ_(y)(ω) can be adjustedby applying the appropriate voltage according to a phase vs. voltagecalibration. Such voltage can be provided by controller 150, which iscalibrated with the necessary pixel phase vs. voltage data, e.g., aslook-up table. As long as elements 166 offer a range of phase variationgreater than 2π, φ_(x)(ω) can be made equal to φ_(y)(ω) and can beprogrammed to any desired value (modulo 2 π).

With reference again to FIG. 1, since PI-SLM 140 in general is made upof an array 145 of independently programmable addressable regions 146,different addressable regions can be programmed via controller 150 toimpart different phases independently using different driving voltages.Combined with the spatial dispersion of frequencies (or equivalently,wavelength) afforded by dispersive module 136, different opticalfrequencies 200 are mapped onto different addressable regions 146. Thisallows for the arbitrary specification of phase vs. frequency, i.e., aprogrammable phase versus optical frequency function that is independentof the SOP of input optical signal 110. Again, calibration of the phasevs. voltage can be readily performed and the data stored (e.g., as alook-up table) so that the precise phase can be encoded onto the signal.

Ideally, one would like to avoid PDL in any of the elements in system100. As the input SOP cannot be specified in system 100, one desireszero PDL, as with any optical processing system where the SOP is notmaintained. However, first dispersive module 136 used for spectraldispersion can have an associated PDL. As PI-SLM 140 of the presentinvention is of the type that does not alter the SOP, one can optionallycompensate for dispersive-module-induced PDL by inserting a half-wave(λ/2) polarization-adjusting element 176 (e.g., a half-wave plate,Faraday rotator, etc.) anywhere in system 100 between dispersive modules130 and 148 (in FIG. 1, element 176 is shown in phantom between PI-SLM140 and dispersive module 148). In a reflective arrangement, half-wavepolarization-adjusting element 176 becomes a quarter-wavepolarization-adjusting element, as discussed below. Further, as element176 can be shown to introduce a simple rotation in polarization incompensated lightwave 126, a second half-wave polarization-adjustingelement (not shown) can optionally be inserted after second dispersivemodule 148 to restore the polarization. A half-wavepolarization-adjusting element gives a 90-degree rotation for thecorrect orientation of axes and a single pass. A quarter-wavepolarization-adjusting element provides 90-degrees of polarizationrotation with two passes for the correct orientation of axes.

With continuing reference to FIG. 1, controller 150 is programmed tocause PI-SLM 140 to selectively and independently adjust the phase (andoptionally the amplitude) of different subsets of spatially separatedfrequency (or equivalently, wavelength) components 200 associated withaddressable regions 146 to produce phase-adjusted, spatially-separatedfrequency components 204. Dispersive module 148 then spatiallyrecombines adjusted frequency components 204 to produce an opticalsignal 126 that is compensated for chromatic dispersion (or, asdiscussed below, restored to its original state of not having anychromatic dispersion).

The phase to be imparted to each frequency component 200 of opticalsignal 110 can be based on information about the chromatic dispersionproperties of a particular optical system (e.g., system 120 or 130) asmeasured or calculated (e.g., based on a model of chromatic dispersioneffects of an optical system). Alternatively, information aboutchromatic dispersion can be acquired empirically by propagating a knownoptical signal (e.g., optical signal 110 or a test signal) through anoptical system and measuring the chromatic dispersion effect.

In an exemplary embodiment of system 100, controller 150 controls PI-SLM140 based at least in part on a feed forward detection signal from adetection system 220, which samples a portion of optical signal 110 tocharacterize its chromatic dispersion. In another exemplary embodiment,controller 150 controls PI-SLM 140 based at least in part on a feedbackdetection signal from detection system 230 that samples a portion ofcompensated optical signal 126 to characterize the effective reductionin the chromatic dispersion from system 100.

Furthermore, in another exemplary embodiment, controller 150 controlsPI-SLM 140 based at least in part on signals from both detection systems220 and 230.

Controller 150 includes the necessary power source and logic forindependently applying electric fields (voltages) to each of respectiveaddressable regions 146. Suitable power sources and logic arecommercially available, e.g., from Cambridge Research andInstrumentation (CRI), Woburn, Mass. Controller 150 can also storeappropriate calibration curves for array 145 so that the voltagenecessary to impart a desired phase retardance is known. The algorithmscan be implemented in computer programs or dedicated integrated circuitsor computer-readable media (e.g., floppy disks or compact disks) usingstandard programming techniques.

Thus, in an exemplary embodiment of the present invention, controller150 includes a computer system 258 (or may be linked to a computersystem) that may be, for example, any digital or analog processing unit,such as a personal computer, workstation, a portion of a console, settop box, mainframe server, server-computer, laptop or the like capableof embodying the programmable aspect of invention described herein. Inan example embodiment, computer 258 includes a processor 260, a memorydevice 262, and a data storage unit 264, all electricallyinterconnected. Data storage control unit 264 may be, for example, ahard drive, CD-ROM drive, or a floppy disk drive that contains or iscapable of accepting and reading a computer-readable medium 268. In anexample embodiment, computer-readable medium 268 is a hard disk, a CD, afloppy disk or the like. Computer-readable medium 268 may containcomputer-executable instructions to cause controller 150 to perform themethods described herein. An example computer 258 is a Dell personalcomputer (PC) or Workstation, available from Dell Computer, Inc.,Austin, Tex.

In another example embodiment, computer-readable medium 268 comprises asignal 270 traveling on a communications medium 272. In one exampleembodiment, signal 270 is an electrical signal and communications medium272 is a wire, while in another example embodiment, the communicationsmedium is an optical fiber and the signal is an optical signal. Signal270 may, in one example, be transmitted over the Internet 276 tocomputer 258 and optionally onward to controller 150.

As described above in connection with system 100 of FIG. 1, controller150 may receive feed forward or feedback signals from detection systems220 and 230, respectively, which characterize the chromatic dispersionin optical signals 110 and 126, respectively. In relatively simpleembodiments with few degrees of freedom, detection system 230 canmonitor the mean pulse dispersion in adjusted optical signal 126 andprovide a detection signal indicative of that dispersion to controller150, which varies the adjustments imparted by PI-SLM 140 to minimize thepulse broadening due to chromatic dispersion (e.g., vary the phaseimparted to each frequency by controlling the voltage provided to eachpixel 146). In more complex embodiments, one or both of detectionsystems 220 and 230 can spectrally characterize the respective lightwavesamples to provide sensing data to controller 150 for each of thespatially separated frequency components 200 incident on PI-SLM 140.

Preferably, one or both of detection systems 220 and 230 sense thespectral phase of the particular optical signal 110 and/or 126 on awavelength-by-wavelength basis. Sensing of the spectral phase (orequivalently the frequency-dependent delay τ(ω)) can be achieved byusing spectral interferometry techniques, cross-correlation techniques,and/or self-referencing measurement techniques, such as frequencyresolved optical gating. Such techniques are described in, e.g., L.Lepetit et al., J. Opt. Soc. Am. B. 12, 2467-2474 (1995), K. Naganuma etal., Opt. Lett. 15, 393-395 (1990), and R. Trebino et al., Rev. Sci.Instrum. 68, 3277-3295 (1997), respectively.

With continuing reference to FIG. 1, dispersive modules 130 and 148 caninclude any dispersive element capable of spatially separating frequencycomponents present in an optical signal. For example, they can include adiffraction grating (e.g., a reflective grating, transmissive grating,an amplitude grating, a phase grating, a holographic grating, echellegrating, arrayed-waveguide grating, etc.), a chromatic prism, and/or avirtually imaged phased array (VIPA). VIPAs are described in, forexample, M. Shiraski, Opt. Lett., 21, 366 (1996), and Shiraski et al.,IEEE Phot. Tech. Lett. 11, 1443 (1999).

Dispersive modules 130 and 148 may further include one or more imagingor relaying optics (e.g., lenses, mirrors, apertures, etc.) fordirecting the frequency components spatially separated by the dispersiveelement in module 130 onto PI-SLM 140 or for directing the adjustedfrequency components from PI-SLM 140 to the dispersive element indispersive module 148. Moreover, in additional exemplary embodiments ofthe present invention, the dispersive modules can be a single opticalelement that combines the dispersing and directing functions, such as adiffractive optical element (DOE).

Even where individual addressable regions of PI-SLM 140 provide manydegrees of control over incident frequency components, the maximumamount of chromatic dispersion that can be compensated or reduced islimited by the spectral resolution of system 100. Generally, theparameters of dispersive module 136 are selected to fully exploit theentire pixel array 145 of PI-SLM 140. In other words, one tries tominimize the range of frequency components 200 on any one pixel 146while also insuring that all frequency components of interest areincident on at least one pixel. Accordingly, spectral resolution can bemade to scale with the number of independently addressable addressableregions 146 of PI-SLM 140.

For example, PI-SLM 140 may have, e.g., at least 2, 4, or 8 addressableregions, and preferably many more, e.g., 64, 128, etc. In any case, toavoid aliasing, spectral variations in the chromatic dispersion of thesignal should be slow compared to the frequency width, denoted δf, ofone pixel 146. This is equivalent to the requirement that the totalduration of the signal to be compensated should be significantly below½δf. The situation may be modified somewhat for embodiments in which thechromatically dispersed optical signal includes multiple signals onseparate wavelength bands. In this case, dispersive modules 130 and 148and PI-SLM 140 can be tailored to optimize spectral resolution withineach band, whereas regions between separate bands may be ignored. Thus,the PI-SLM can have multiple sets of arrays 145, with each arraydedicated to a particular wavelength band.

With continuing reference again to FIG. 1, dispersive module 136 andPI-SLM 140 combine to function as a programmable spectral phaseequalizer by independently adjusting the phase of optical signal 126 ona wavelength-by-wavelength basis. The approach allows compensation oftime-varying chromatic dispersion effects, at least down to the responsetime of PI-SLM 140. For a nematic liquid-crystal based PI-SLM (FIG. 2A),this response time is on the order of tens of milliseconds, which isfast enough to handle the majority of effects that cause chromaticdispersion

It is worth remarking on the relationship between delay and spectralphase. For complete phase control, PI-SLM 140 only needs to vary thephase at each pixel 146 over a 0-2π radian range, which by itselfconstitutes a small phase delay. The frequency dependent group delay,however, varies as the derivative of phase with respect to frequency. Inparticular, frequency-dependent delay π(ω) is related to a spectralphase variation ψ(ω) as shown in EQ. 4: $\begin{matrix}{{\tau(\omega)} = {- \frac{\partial{\psi(\omega)}}{\partial\omega}}} & \left( {{EQ}.\quad 4} \right)\end{matrix}$Therefore, even relatively large group delays that may be associatedwith chromatic dispersion, e.g., in the tens of picoseconds range, canbe compensated using physical phase delays no larger than 2π. Forvisible and near infrared wavelengths, such phase delays correspond to aphysical phase delay of only a few femtoseconds.

EXAMPLE EMBODIMENTS

As mentioned above, there are many specific examples of system 100 ofFIG. 1. Several of these examples are described below for the sake ofillustration, and one skilled in the art will appreciate that theexamples provided in no way limit the general teaching of the chromaticdispersion compensation system of the present invention.

Transmission System with Liquid Crystal PI-SLM and Diffraction Gratings

Referring now to FIG. 3, a first exemplary embodiment of system 100 isshown. In this embodiment, first dispersive module 136 includes a firstgrating 300 for receiving optical signal 110 and angularly dispersingits frequency components 200, and a first lens 306 having a focal lengthF1 for collimating the angularly dispersed frequency components andfocusing them onto a liquid-crystal-based PI-SLM 140, and in particularonto first and second arrays 160 and 162 of elements 166 (FIG. 2A)

Optical signal 110 emanates from the end of an output optical fiber 122as part of optical system 120 and is incident on first grating 300. Thecollimation and focusing of frequency components 200 can be accomplishedby spacing lens 306 from each of grating 300 and PI-SLM 140 by adistance equal to its focal length F1. Thus, the grating and lens mapthe frequency content (i.e., components 200) of optical signal 110 ontoSLM arrays 160 and 162. Moreover, because of the positioning of lens306, grating 300, and PI-SLM 140, the spatial extent of any individualfrequency component on arrays 160 and 162 is minimized. For each pixel,PI-SLM 140 independently adjusts the phase (and optionally theamplitude) of the frequency components 200 incident on the pixel (inFIG. 2A, only two frequency components 220 are shown for simplicity).

Second dispersive module 148 includes a second grating 320 and a secondlens 326 having a focal length F2 for recombining the adjustedspatially-separated frequency components 204 into adjusted opticalsignal 126, which can then be coupled to an optical fiber 132 as part ofsecond optical system 130. Like first dispersive module 136, lens 326 ispreferably spaced from each of PI-SLM 140 and grating 320 by a distanceequal to its focal length F2. In an example embodiment, the focal lengthof lenses 306 and 326 are the same (i.e., F1=F2=F), and thus thegratings, lenses, and SLM define a “4-F” arrangement.

An advantage of the present invention is that gratings 300 and 320 neednot be polarization insensitive, since system 110 as a whole does notrely on knowledge of SOP of optical signal 110.

In other embodiments of system 100 of FIG. 3, for example, lenses 306and/or 326 can be replaced with curved mirrors having a radius ofcurvature equal to 2F, in which case the arrangement is folded.Similarly, the arrangement can be folded by using a reflective PI-SLM,as discussed below. Also, transmission gratings may be used instead ofreflective gratings 300 and 320. One skilled in the art will appreciatethe basic equivalency between folded reflected systems and unfoldedtransmission systems. Moreover, in additional embodiments, thedispersive modules and PI-SLM may be implemented, in whole or in part,as an integrated waveguide structure.

Depending on the nature of gratings 200, PDL can be significant. Thus,optionally included in system 100 of FIG. 3 is half-wavepolarization-adjusting member 176 shown in phantom just downstream ofPI-SLM 140, in order to reduce any PDL from gratings 200 and 320.

On-Axis Reflection System with Diffraction Gratings

With reference now to FIG. 4, an on-axis reflective system 100 isillustrated. System 100 of FIG. 4 is similar to the transmissive systemof FIG. 3 in that it is an optically folded version thereof. Inparticular, system 100 of FIG. 4 includes a single optical fiber 122serving as both the input and output fiber. This is made possible byconnecting optical fiber 122 to a circulator 400 to which is alsoconnected a first optical fiber 406 and a second optical fiber 408. Achromatically dispersed input optical signal 110 is provided by firstoptical fiber 406, and is passed to optical fiber 122 by circulator 400.Optical signal 110 is dispersed by grating 300 into its constituentfrequency components 200 and imaged by lens 306 onto active area 144 ofreflective PI-SLM 140. Addressable regions 146 of reflective PI-SLM 140are programmed to impart the appropriate phase for each frequency, asdescribed above, to create dispersion-compensated signal components 204and reflect the components through lens 306 and to grating 300. A ¼-wavephase plate 410 (shown in phantom) is optionally provided to provide atotal of ½-wave (i.e., 90-degrees) of total polarization rotation overtwo passes of the light through the plate, to limit PDL, as discussedabove. The combination of lens 306 and grating 300 serves to recombinethe frequency components to form compensated optical signal 126 andrelay the optical signal back to optical fiber 122. Optical signal 126propagates along optical fiber 122 until it encounters circulator 400,which directs optical signal 126 to second optical fiber 408.

On-Axis Reflection System with Magnification

With continuing reference to FIG. 4, in order to disperse a givenwavelength band across active area 144 of PI-SLM 140, lens 306 needs tohave a certain focal length F1. The larger PI-SLM 140 and the narrowerthe wavelength band of optical signal 110, the longer focal length F1must be. For many diffraction gratings, F1 must be on the order of 10 mto disperse a 1 nm band across a 1 cm active area 144. This makes for avery long optical path for system 100.

Accordingly with reference now to FIG. 5A, a compact on-axis reflectivesystem 100 is illustrated. System 100 of FIG. 5A is similar to thereflective system of FIG. 4, except that magnification is introduced toshorten the system. Note that reflective designs by their nature aremore compact than transmissive designs, and also tend to be moreeconomical because the components can be employed to perform “doubleduty” by passing light through select components in two differentdirections.

Magnification is achieved in the present invention by forming anintermediate image I1 at an intermediate image plane P1 of spectralcomponents 200 formed by grating 300 located at a plane P0. Image I1 isthen used as an object for forming a magnified image I2 of the frequencycomponents 200 at a second image plane P2 coincident with active area144 of PI-SLM 140 using a magnifying optical system 460 arranged betweenplanes P1 and P2. Thus, magnifying optical system 460 relays withmagnification frequency components 200 onto active area 144. In general,the magnification provided is such that the optical path of system 100is shortened as compared to the optical path without the introduction ofmagnification. The necessary magnification will depend, in part, on thesize of active area 144 of PI-SLM 140 and the amount of dispersion ofthe frequency components provided by dispersive module 136.

With reference to FIG. 5B, in an example embodiment optical system 460includes a telescope with a first lens 470 having a focal length F4 anda second lens 476 having a focal length F5. Lens 306 has a focal lengthF1. In a preferred embodiment, the distance between planes P0 and P1 is2F1 and the distance between plane P1 and P2 is set to 2(F4+F5). Oneskilled in the art will appreciate that the judicious arrangement andchoice of lens 306 and the elements making up optical system 460 cansignificantly reduce the length of system 100 should the systemotherwise prove to be too long for the particular application. Also, themagnification technique used in this reflective embodiment as an exampleis equally applicable to a transmissive system (e.g., system 100 of FIG.2A).

Because the size of system 100 scales with the size of the active area144 of PI-SLM 140, it can also be made compact by using an SLM with asmaller active area (i.e., aperture) 144 and smaller addressable regions146 in addition to, or as an alternative to providing magnification. Forexample, certain liquid crystal SLMs have addressable regions (pixels)of typically about 100 microns, but also as small as 25 microns, whichallowing 512 pixels to fit into a 12.8 mm aperture. Such an SLM isavailable from the Raytheon Company in Lexington, Mass. A similar SLMwith 128 pixels would have an aperture of approximately 3 mm. A liquidcrystal SLM from Boulder Nonlinear Systems, Boulder, Colo., has 4096pixels, with a center-to-center pixel spacing of 1.8 microns and anaperture of 7.4 mm.

Thus, a small PI-SLM 140 has an aperture size of about 5 mm across, andin an example embodiment, has an aperture size of 3 mm or less. With areflective system 100, using a PI-SLM having an active area of (3 mm×3mm) and a magnification of 30 can result in system 100 having an overalllength as small as, for example, 40 cm, for a 1 nm optical bandwidth.This system could be made more compact by folding the optical pathusing, for example, fold mirrors or fold prisms.

Off-Axis Reflection System

With reference now to FIG. 6, there is shown another example embodimentof system 100 of the present invention that utilizes an off-axisreflective design. System 100 of FIG. 6 is similar to that of FIG. 4,except that reflective PI-SLM 140 is tilted relative to axis A1, so thatthe return path of light rays 480 from PI-SLM associated withcompensated optical signal 126 are not coincident with the incident pathof light rays 490 associated with incident optical signal 110. Thisallows for different input and output fibers 112 and 132 to be used,rather than a single fiber.

Optical Processing Systems Implementing the System 100

System 100 can also be implemented in optical processing systemconfigurations other than that shown in FIG. 1. In particular, ratherthan compensating or reducing chromatic dispersion in an optical signalafter it has passed through the optical system, system 100 can be usedto pre-compensate an optical signal prior to its transmission through anoptical system having chromatic dispersion. Furthermore, in addition topost-compensation and pre-compensation, system 100 can be used in animplementation that interrogates the optical system having chromaticdispersion, rather than sensing or detecting the actual optical signalitself. In addition, system 100 can be used to perform independentchromatic dispersion control of optical signals for independent WDMchannels.

These various implementations are now described in greater detail below.

Post-Compensation Implementation

With reference now to FIG. 7A, there is shown a source 510 for providingan undistorted optical signal 516, which passes through an opticalsystem 520 having chromatic dispersion to produce a chromaticallydispersed optical signal 530, akin to optical signal 110 of FIG. 1.Optical signal 530 then passes through system 100 (e.g., as showngenerically in FIG. 1 or any of the specific embodiments thereofdiscussed above in the ensuing Figures) to reduce the chromaticdispersion in the signal and produce an adjusted optical signal 126.Signal 126 is detected by detection system 230 to monitor the degree ofcompensation and allow for iterative measurements and compensations toprovide an optimally reduced chromatic dispersion. Signal 530 may alsobe detected by detection system 220 to determine the amount ofcompensation needed to be applied to signal 530 by system 100.

Pre-Compensation Implementation

With reference now to FIG. 7B, there is shown a pre-compensationimplementation of chromatic dispersion compensation system 100. Inparticular, undistorted optical signal 516 first passes through system100 that is adapted to alter signal 516 in a predetermined manner tocounteract the anticipated effects of downstream optical system 520. Theresult is an optical signal 550 that includes a predetermined amount ofchromatic dispersion. Optical signal 550 then passes through opticalsystem 520. The chromatic dispersion imparted to signal 550, however,was selected to offset or reduce the impact of the chromatic dispersioncaused by optical system 520. Thus, an optical signal 516′ emerges fromoptical system 520 having reduced, if not fully compensated, chromaticdispersion. Accordingly, optical signal 516′ closely resembles signal516, if not identical thereto. In an exemplary embodiment, chromaticdispersion compensator 100 is guided by detection system 230, whichprovides a precompensation signal to controller 150 representative ofthe state of chromatic dispersion of optical signal 516′, which isindicative of the chromatic dispersion effects in downstream opticalsystem 520.

Optical System Compensation Implementation

With reference now to FIGS. 7C and 7D, there are shown opticalprocessing systems involving post-compensation and pre-compensationimplementations wherein the compensation of the optical signal isdetermined by interrogating optical system 520 directly, rather than bysensing or detecting an optical signal using detection systems (sensors)220 and/or 230.

Accordingly, a sensor 580 is arranged to be in optical communicationwith optical system 520 and system 100, wherein the sensor is adapted tosense the chromatic dispersion of the optical system. This may carriedout, for example, by providing one or more lightwave test signals 586having particular characteristics (e.g., a set bandwidth, pulse lengthand/or pulse shape) through optical system 520, and measuring the amountof chromatic dispersion induced using system 100. System 100, viacontroller 150, processes the measurements and determines the amount ofpre-compensation (FIG. 7D) or post-compensation (FIG. 7C) for chromaticdispersion is required for system 520. Once the amount of compensationis determined, then the actual input signal 516 can be emitted fromsource 510.

WDM Optical Processing System Implementation

Although the preceding paragraphs refer to compensation of pulsebroadening caused by chromatic dispersion, it is noted that opticalsignals 110 and 126 (FIG. 1) may carry such pulse information on one ormore different wavelength bands (channels). Thus, in one limit, theentire frequency bandwidth of the optical signal may be used to carryhigh-bandwidth, pulsed information (e.g., time-domain multiplexing orTDM), whereas, in the opposite limit, the frequency bandwidth of theoptical signal is divided into separate wavelength bands, eachsimultaneously carrying lower-bandwidth pulsed information (e.g.,wavelength-division multiplexing or WDM).

Thus, with reference to FIGS. 8A and 8B, there is shown an opticalprocessing system-level view of a system implementation that allows forindependently compensating the optical signal associated with differentWDM channels (“channel optical signals”). The system includes upstreamoptical system 120 and a multiplexed optical signal 604 comprisingchannel optical signals 110 a, 110 b and 110 c centered at wavelengthsλa, λb and λc, respectively, and each having a wavelength band Δλ. In anexample embodiments, the channel spacing is 0.8 nm (i.e., 100 GHz), thewavelength band Δλ=0.1 nm (i.e., 12.5 GHz) λa, λb and λc=1550 nm, 1550.8nm and 1551.6 nm. Three channel optical signals are used for the sake ofillustration; clearly, greater or fewer channels optical signals can beused.

Channel optical signals 110 a, 110 b and 110 c pass through ademultiplexer 600, which separates the channel signals so that they canbe coupled into corresponding optical fibers 610 a, 610 b and 610 c.Each of fibers 610 a, 610 b and 610 c is coupled to a system 100.Systems 100, as described above, each adjust the phase of the frequencycomponents of the corresponding channel optical signal so that thesignal is compensated for chromatic dispersion, as described above. Theresult is compensated channel signals 126 a, 126 b and 126 c travelingalong optical fibers 610 a, 610 b and 610 c

With reference now to FIG. 8A, compensated channel signals 126 a, 126 band 126 c are then received by respective receiving optical systems 130,which in the present embodiment may simply be optical detectors thatdetect the optical signal and convert it to an electrical signal.

With reference now to FIG. 8B, the optical processing system of thepresent implementation may further include a multiplexer 620 thatmultiplexes compensated signals 126 a, 126 b and 126 c to form acompensated WDM signal 628, which can then be passed along to opticalsystem 130. The optical processing systems of FIGS. 8A and 8B can beused in either the pre-compensation or post-compensation modes.

WDM Compensation Using a Single PI-SLM

With reference now to FIG. 9, there is shown a close-up view of aportion of system 100 of FIG. 1, wherein multiplexed optical signal 604comprising channel optical signals 110 a, 110 b and 110 c centered atwavelengths λa, λb and λc, respectively, and each having a wavelengthbandwidth Δλ. Three channel optical signals are used for the sake ofillustration; clearly, greater or fewer channels optical signals can beused.

Multiplexed optical signal 604 is incident on dispersive module 136. Thelatter is designed to disperse not only the frequency components 200a-200 c of the individual channel optical signals 110 a, 110 b and 110c, but also to disperse the different channels signals relative to oneanother. Thus, the different channel optical signals 110 a-110 c ofmultiplexed signal 604 are dispersed to different zones 630 (e.g., 630a-630 c) on PI-SLM 140, with each zone containing a set of addressableregions 146. Thus, the addressable regions 146 in zones 630 a-630 c arededicated to compensating the optical frequencies in the wavelength bandΔλ centered around each wavelength λa-λc, respectively. This allowsdifferent channel optical signals centered at different wavelengths tobe independently and simultaneously compensated and optimized. This isadvantageous because chromatic dispersion properties vary across thewavelength spectrum, so that different optical signals centered arounddifferent wavelength bands will generally require different amounts ofchromatic dispersion compensation.

It should be noted that this WDM chromatic dispersion correctionapproach requires a trade-off between the number of addressable regionsper channel optical signal and the number of channel optical signals inthe WDM signal. In an example embodiment, the number of addressableregions per channel optical signal is preferably between about 8 and 16.Further, the total number of addressable regions is preferably about 128or greater.

Conclusion

The invention provides the capability to programmably control pulsebroadening due to chromatic dispersion in optical fiber communicationand networking systems. This capability allows optical fiber lightwavecommunication systems to run at higher speeds over longer distances bycompensating chromatic dispersion, which is regarded as a key impairmentfor high performance lightwave communication systems.

The present invention can be applied both to very high-speedtime-division multiplexed (TDM) and to wavelength division multiplexed(WDM) optical communications. In the case of WDM systems, several WDMchannels can be compensated independently and can be programmed toachieve nearly arbitrary dispersion profiles in order to match thesystem requirements. The chromatic dispersion compensation system of thepresent invention can handle input optical signals with an arbitrary andunspecified SOP, and may be configured to provide substantially zeroPDL. The programmable nature of the present invention allows forchromatic dispersion to be compensated under a variety of conditions andsituations, including re-programming the chromatic dispersioncompensation system when there is a change in the optical processingsystem configuration (e.g., the length of the network) that results in achange in the system chromatic dispersion.

A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention. Forexample, some embodiments may incorporate both pre-compensation andpost-compensation. In such case, a pre-compensation system may be usedto match the wavelength-by-wavelength chromatic dispersion of an opticalsignal to the wavelength-by-wavelength chromatic dispersion of adownstream optical system. Thereafter, a post-compensation system canfurther reduce chromatic dispersion in the optical signal caused by theoptical system

Thus, while the present invention has been described in connection withpreferred embodiments, it will be understood that it is not so limited.On the contrary, it is intended to cover all alternatives, modificationsand equivalents as may be included within the spirit and scope of theinvention as defined in the appended claims.

1. An optical processing system for altering the phase of the frequencycomponents of channel optical signals traveling in different WDMchannels so as to reduce chromatic dispersion effects in the channeloptical signals, comprising: a wavelength division demultiplexer fordemultiplexing the channel optical signals; a plurality of opticalfibers connected to the wavelength division demultiplexer each forcarrying one of the demultiplexed channel optical signals;polarization-independent chromatic dispersion compensation meansarranged in each optical fiber for independently performing chromaticdispersion compensation of each channel optical signal;
 2. A systemaccording to claim 1, further including receiving-optical systemsarranged adjacent each chromatic dispersion compensation means, forreceiving corresponding compensated channel optical signals.
 3. A systemaccording to claim 1, further including a wavelength divisionmultiplexer connected to the optical fibers for multiplexing thecompensated optical signals.
 4. A system according to claim 3, furtherincluding a receiving-optical system optically coupled to the wavelengthdivision multiplexer for receiving the compensated multiplexed signal.5. A system for programmably adjusting the phase of the frequencycomponents of an optical signal of arbitrary polarization to compensatefor chromatic dispersion in the optical signal, comprising: means fordispersing the frequency components of the optical signal; programmablephase-adjusting means for programmably adjusting the phase of one ormore of the frequency components; and means for combining thephase-adjusted frequency components to form a compensated opticalsignal.
 6. A system according to claim 5, further including controlmeans for controlling the operation of the phase-adjusting means.
 7. Asystem according to claim 5, further including computer means forproviding phase-adjusting information to the control means.